Graphene is black

Graphene can be saturated readily under strong excitation over the visible to near-infrared region, due to the universal optical absorption and zero band gap. Wiki

So this has relevance to artifical photosynthesis. We can use carbon nanotube sheets which passes water under sunlight. These sheets act like an prosthetic for photosynthesis leaving the OH- and H+ available for recombination. We can imagine a special, disabled, algae engineered that live just underneath these sheets exposed in sunlight while bathed in ocean water.

Like this:

In the work reported in Science, the scientists added the acid indirectly by bubbling carbon dioxide into the water, which more closely mimicked the chemical reactions that are occurring in the oceans. As a consequence, in addition to the lowered pH, levels of carbon dioxide in the water also rose — speeding up the algae’s photosynthesis machinery — as did the levels of bicarbonate ions NYT

So the idea is that proper energy gap material would nominally create various acids on its underside, algae use recombination with acid reactions.